Salmonella are prominent intestinal pathogens that cause both enteritis and systematic disease. The overall goal of this unit is to utilize the resources of the Program Project in an interactive approach to elucidate key aspects of Salmonella pathogenesis in the intestinal tract as well as in systemic tissues. Salmonella represents an excellent model for an invasive intestinal pathogenesis in the intestinal tract as well as in systemic tissues. Salmonella represents an excellent model for an invasive intestinal pathogen since the organism is amenable to detailed molecular genetic analysis. Specific Aim 1 is to identify S. typhimurium genes that are important for infection of human intestinal epithelial cells and stimulation of pro-inflammatory epithelial cell responses. This approach will utilize the expertise of Dr. Kagnoff's laboratory in epithelial cell signaling in response to mucosal infections. Specific mutations in Salmonella genes governing attachment invasion and intracellular growth will be tested for effects on human epithelial cells in culture and examined in vivo using human intestinal xenografts in SCID mice. Specific Aim 2 is to isolate rpoS-regulated genes in S. typhimurium and determine their virulence phenotypes. This proposal is directly derived from work showing that the alternative sigma factor sigmas (RpoS) is a prominent regulatory element during Salmonella infection of host cells, including intestinal epithelial cells. rpoS-regulated genes will be identified by a molecular genetic approach, and virulence will be tested in mice as well as in models of human intestinal infection. Specific Aim 3 is to determine the roles of polymorphonuclear leukocytes in a) natural resistance to oral infections by Salmonella of different O antigen types, b) protecting mice that have defective macrophage function (a mutant Nramp 1 allele) from Salmonella infections; and c) acquired immunity (antibody mediated) to Salmonella. This aim will complement work on the ability of epithelial cells to initiate the pro-inflammatory host response, and will focus on crucial host defenses that limit the spread of infection from the intestine to systemic sites.